1. Field of the Invention
The present invention relates to refrigeration circuits. More specifically, the present invention concerns the utilization of the current to an electric motor driving a variable capacity compressor to determine when to increase the capacity of the compressor.
2. Prior Art
To effectively utilize an air conditioning system it is desirable to match the compressor output to the load on the system. Matching compressor output to the load on the system has been accomplished in many ways. One way is to operate the compressor motor at separate speeds thereby pumping separate amounts of refrigerant at each speed. Another way is to use valve unloaders and bypass means to limit the number of cylinders effectively pumping refrigerant within the compressor. A hot gas bypass wherein some of the discharge gas is circulated back to the compressor suction is another method of limiting compressor output. In centrifugal compressors, guide vanes are utilized to control the flow of refrigerant gas into the compressor to regulate the output by controlling the input.
The present invention is particularly concerned with a reciprocating type compressor capable of having varying refrigerant outputs in discrete stages. These outputs are controlled via unloader valves which effectively operate to render inoperative, in terms of pumping refrigerant, at least one of a pair of reciprocating pistons. To more effectively regulate the flow of refrigerant from the compressor, these individual pistons may be chosen to have varying displacements such that rendering one inoperative reduces refrigerant flow by a substantially different amount then rendering the other inoperative. Via this arrangement, a compressor having three capacity steps may be achieved by having two varying sized pistons. For a complete description of such a compressor and the control system therefor, please see U.S. patent application Ser. No. 479,044, entitled "Variable Volume Compressor And Method Of Operating", filed Mar. 25, 1983.
In split system air conditioning units, the compressor and condenser are typically located remote from the indoor heat exchanger. In such a system it would be advantageous, in terms of energy consumption, to have a multiple capacity compressor. In split systems having multiple indoor heat exchangers serviced by a single compressor and a single condenser, the advantages of utilizing a variable capacity compressor are further increased. Such a system might typically include three indoor heat exchangers connected to a single compressor and a single condenser. The number of operating stages of the compressor could be matched to the number of indoor heat exchangers such that the load on the system may be balanced simply by selecting the appropriate stage of the compressor for the number of heat exchangers being operated.
Such a system, however, is overly simplistic and, depending upon the various operating conditions of the separate indoor heat exchangers, may result in the compressor working too hard and wasting energy or being at a capacity stage which is sufficient to meet the load on just a partial number of indoor coils. For instance, should the outdoor ambient temperature be extremely high and only two indoor coils be calling for cooling (the third being shut down because the space is not being utilized) the compressor may need to operate in its highest capacity step as opposed to a lower capacity step to satisfy the load on just two indoor coils.
On the other hand, should the outdoor ambient temperature be relatively low and all three indoor fan coils are calling for cooling because of humidity conditions of the spaces being occupied, then the operation of the compressor at its highest capacity step may not be required to meet the cooling load.
The current device as disclosed herein utilizes capacity pressure sensors to determine when pressure levels have been reached. Specifically, a heating capacity pressure sensor is utilized and is connected to the compressor discharge line to sense the discharge pressure from the compressor. The heating capacity pressure sensor uses a switch arranged to move from a first state to a second state upon the pressure level being sensed exceeding a predetermined value. Hence, when the compressor discharge pressure exceeds the predetermined level of the heating capacity pressure sensor, the sensor changes from a first state to a second state indicating a need to reduce the compressor capacity. To reset the heating capacity pressure sensor, the sensor is subjected to low pressure to change the sensor from the second state back to the first state. The sensor is now in position to detect another variation above the preset pressure level. Between the heating capacity compressor sensor tripping and before the pressure sensor is again connected to sense the discharge pressure, the capacity of the compressor is reduced. As outlined in this herein application, a three state or three capacity step compressor is disclosed. If the compressor is operating at high capacity and the heating capacity pressure sensor indicates too much capacity- is present, the compressor will be cycled to the next lower or midlevel capacity.
A cooling capacity pressure sensor may also be utilized being set to trip upon the suction pressure to the compressor falling below a predetermined level. This sensor works similarly to the heating capacity pressure sensor in that upon the pressure falling below the predetermined level it changes from a first state to a second state. The capacity step at which the compressor is operated is decreased in response to the sensor tripping and the sensor is then reset by exposing the sensor to the relatively high discharge pressure from the compressor for a short interval.
In order to increase the capacity of the variable capacity compressor different means are used. One method is to monitor the value of the current being supplied to the compressor motor driving the compressor. After initialization, a current reference value is determined. Thereafter, at predetermined time intervals, the value of the current actually being drawn by the compressor is compared to the reference value. In the cooling mode of operation, should the value of the compressor current actually being monitored exceed the value of the reference current by a predetermined amount then it appears that an increase in capacity is required. The logic is provided to then increase the capacity step at which the compressor operates. In the heating mode of operation, the current comparison logic is similar. In this mode of operation, the value of the actual current is compared to the value of the reference current and whenever the actual current is a certain factor less than the value of the reference current and it is apparent that the capacity step of the compressor should be increased.
Additional means for increasing the capacity step of the compressor may also be provided. One such means would include automatically increasing the capacity step to the next step or to the maximum step upon the addition of any load to the refrigeration circuit. An additional load might be the energization of an additional heat exchanger of the three as set forth in this particular system.
Another method of determining when to increase the capacity step is to monitor the length of time the compressor continuously operates at a given capacity level. If this length of time should exceed a chosen set point then the compressor capacity could automatically be increased to the next level. A time period such as thirty minutes would be appropriate.